Conventional optical fibers generally used comprise two layers: a core for light confinement, and a cladding circumferentially covering the core, where the cladding has a slightly lower refractive index than that of the core, and the core and cladding are both formed of quartz material. Because the refractive index of the core is slightly higher than that of the cladding, the two-layered fibers allow light to be confined in the core by the refractive index difference, and to thereby propagate in the optical fibers.
Regarding methods for connecting single mode fibers to each other, there are connection methods by a connector and a mechanical splice. The connector connection method mounts optical fibers to respective optical connectors in such a manner as to be easily detachable. The mechanical splice connection method joins the end faces of optical fibers to each other in a V-groove or the like provided in the mechanical splice in such a manner as to securely hold both of the joined optical fibers. The connection techniques for typical single mode fibers have been fully developed.
Photonic crystal fibers (PCFs) have recently been remarkable.
PCFs are optical fibers having a photonic crystal structure in its cladding, i.e., a periodic refractive index structure. Light can be localized by reducing the periodic structure up to the order of light wavelength or several times the light wavelength to introduce a defect and local nonuniformity into a crystal.
Referring to FIG. 5, there is explained a cross-sectional structure of this PCF.
A PCF 41 is formed of only a cladding 42 whose refractive index is all the same in the fiber, from the center of which are arranged a multiplicity of cylindrical air holes 43 in a hexagonal lattice form, where the cylindrical air holes 43 extend along the entire length of the fiber 41. The member having a light confinement function, which corresponds to a conventional core, is a crystal defect portion 44 at the center of the fiber 41.
Specifically, in a pure quartz fiber with a cladding diameter φ of 125 μm, the cylindrical air holes 43 with a diameter φ of 3 μm are arranged in the cladding 42 from the center periodically in a hexagonal lattice form (4 periodic structure), and no air holes are formed at the center (crystal defect), which serves as the core 44 for light confinement.
A technique is essential for connecting a PCF which has a large light confinement effect, and a single mode fiber (SMF) now used in long-distance large-capacity communications.
Japanese patent application laid-open No. 2002-243972 discloses a method for connecting a PCF and an SMF, in which a PCF end to be connected is heated for mounting to a ferrule.
However, the above connection method can be applied to only a PCF whose core is formed of a higher-refractive-index medium than that of its cladding. In other words, the above connection method cannot be applied to a fiber structure whose core and cladding have the same refractive index, and in which a photonic crystal structure with cylindrical air holes allows light to be confined in the core by equivalently providing a refractive index difference between the core and cladding. This is because heating the PCF end to be connected causes fusion bonding of the wall of the cylindrical air holes so that the cylindrical air holes vanish, which therefore results in no core. In this case, the cores of the PCF and the SMF to be connected thereto are connected to each other via a portion in which no core is present, which therefore results in an increase in connection loss.
Accordingly, it is a first object of the present invention to provide a PCF-type optical fiber, a method for connecting the PCF-type optical fiber and an SMF, and an optical connector, which are capable of suppressing an increase in connection loss.
In Holey fibers (HFs) which are a kind of PCFs, on the other hand, air holes are formed in a cladding around a core of a conventional optical fiber, to reduce the effective refractive index of the cladding to increase the relative refractive index difference between the core and cladding, thereby allowing greatly enhancing its bend loss property, compared to the conventional optical fiber. (See, “A study on practical use of Holey fibers”, Yao-B, et al., Shingaku Giho, Institute of Electronics, Information and Communication Engineers, Vol. 102, No. 581, pp. 47-50; “Trend of the development of photonic crystal fibers and Holey fibers”, Takemi Hasegawa; Monthly periodical “Optoelectronics”, Optoelectronics Inc., No. 7, pp. 203-208 (2001)).
In such an HF with a plurality of air holes extending in the axial direction of the fiber in its cladding, if these air holes are open-ended, moistures invade thereinto, which would cause a degradation in mechanical strength, and a variation in optical properties due to dew condensation caused by temperature variations.
As methods for obviating such problems, Japanese patent application laid-open No. 2002-323625 discloses methods for sealing air holes of an optical fiber, (1) by using a fusion splicer (apparatus for joining optical fibers by fusing the optical fibers by gas discharge) to heat the end face of the optical fiber to soften its cladding to collapse the air holes; (2) by inserting a hardened substance into hollow portions; and (3) by fitting a lid onto hollow portions from outside.
However, in the (1) method, because cladding material around the air holes is fused to fill and thereby seal the air holes but the amount of the material does not change, the diameter (cladding diameter) of the optical fiber becomes small. For instance, in the case of a cladding diameter of 125 μm and 4 air holes with a diameter of 10 μm, with simple calculations, the cladding diameter is reduced by approximately 2 μm to the order of 123 μm. This amount becomes larger as it is more different from the inside diameter of a standard ferrule in typical connector connection. Also, when the heating source is a fusion splicer, because of a high temperature of its discharge gas and also effects of evaporation in the cladding surface, the actual dimensions are still smaller, and in the case of a larger total cross-section of the air holes, the dimensions are more remarkably reduced. This causes a time-consuming choice of a ferrule matching a cladding diameter after sealing. In addition, because the discharge gas heats the optical fiber end face and cladding surface therearound together, which makes an edge of the optical fiber end round, there is the drawback that the dimensions around the end face tend to vary.
In the (2) method, because hardening of the hardened substance is accompanied by volume contraction, foams occur in hardened portion. The space inside the foams has a refractive index of approximately 1, which therefore makes the refractive index difference between the hardened substance and the foams very large, and if portion with such a large refractive index variation is adjacent to the core, it affects waveguide structure of the optical fiber, which would cause a large loss.
In the (3) method, there is the drawback of notable variations in the dimensions around the end face.
Accordingly, it is a second object of the present invention to provide a structure and method for sealing an end of an optical fiber, which are capable of maintaining dimensions around the end face, with a cladding diameter maintained accurately, without affecting waveguide structure of the optical fiber.
In the above-described Holey fiber, on the other hand, during connector processing, direct grinding of an end face would cause ground powder and abrasive to invade into air holes of the fiber and remain therein even after connector processing. When the connector is repeatedly attached and detached, the remaining ground powder and abrasive can be released from the air holes in such a manner as to adhere to the ground surface of the fiber. Connector connection with ground powder and abrasive adhering to the ground surface of the fiber prevents close contact of the connector end faces, which would cause not only an increase in loss, but, in the worst case, also concern for damaging the ground surface so that even if the end face is cleaned, the increased loss is not recovered.
Accordingly, it is a third object of the present invention to provide an optical fiber and an optical fiber connector, which are capable of low-loss connection with no remaining ground powder and abrasive caused in air holes of a fiber end face during grinding, and which are also excellent in long-term reliability.
A Holey fiber (HF) is explained in detail again. FIG. 17 illustrates an HF 361 comprising a core 362 made of germanium-added pure quartz, a cladding 363 of pure quartz formed therearound, and a plurality of air holes 364 (6 air holes in FIG. 17) extending axially so as to surround the core 362 in the cladding 363. Although not illustrated in detail, the HF 361 is used as an optical fiber core wire with a coating layer formed around the cladding 363.
The core 362 is the same as a core of typical single mode fibers (SMFs). The core diameter φ is 9 μm, the cladding diameter φ 125 μm, and the air hole 364 diameter φ 8 μm. The refractive index of the core 362 is 1.463, and the refractive index of the cladding 363 is 1.458, and the relative refractive index difference of the core 362 to the cladding 363 is approximately 0.35% which is the same as that of typical SMFs.
The features of the HF 361 are as follows: The refractive index of the air holes 364 is 1, and the effective relative refractive index difference is approximately 32% which is much larger than that of typical SMFs, which therefore has a large light confinement effect to the core 362. For this reason, the HF 361 has an extremely small loss caused when the HF 361 is bent, for example.
FIG. 18 illustrates a conventional optical fiber connection portion 370 in which a coating-removed and end-treated end face 361a of the HF 361 is joined to a coating-removed and end-treated end face 371a of the SMF 371 via a gelled refractive index matching agent r7. The SMF 371 comprises a core 372 with the same refractive index and diameter as those of the core 362 of the HF 361 and a cladding 373 with the same refractive index and diameter as those of the cladding 363 of the HF 361.
Because an air layer can be formed between respective end faces 361a and 372a of the HF 361 and the SMF 371 due to error in end treatment after end-to-end joining, the refractive index matching agent r7 is used to reduce a Fresnel reflection loss due to a refractive index difference caused by this air layer.
The refractive index matching agent r7 has a temperature characteristic which obeys a temperature characteristic line 381 of FIG. 19, for example. In order to make a Fresnel reflection loss as small as possible, the refractive index of the refractive index matching agent r7 is around 1.463 at around room temperature, which is substantially equal to the refractive index of the respective cores 362 and 372 of the HF 361 and the SMF 371 explained in FIG. 18. Further, refractive indices vary according to wavelengths, which, unless otherwise noted herein, are measured values indicated by nD25, i.e., measured values at 25° C. using Na D-line (wavelength 587.56 nm).
As one example of conventional optical fiber splicers with the optical fiber connection portion 370 housed therein, there is a single-core mechanical splice 391 as illustrated in FIG. 20 (see, e.g., Japanese patent application laid-open Nos. 2000-241660, and 2002-236234). The mechanical splice 391 comprises a V-groove substrate 392 having a V-groove for end-to-end joining, supporting, positioning and core alignment of optical fibers facing each other; a lid 393 for being superimposed on the substrate 392 to hold the optical fibers inserted into the V-groove; and sandwiching members 394 for sandwiching the substrate 392 and the lid 393 therebetween.
In superimposed portions of the substrate 392 and the lid 393, wedge-inserting portions 395 are formed at their side, and guild holes 396 are formed at both their ends respectively. A chassis 397 comprises the substrate 392 and the lid 393.
In the mechanical splice 391, an end-to-end joining position of the optical fibers (an inner-surface middle portion of the substrate 392 and the lid 393) is beforehand filled with a refractive index matching agent r7 explained in FIGS. 18 and 19. Wedges are respectively inserted into the wedge-inserting portions 395 so as to form a gap between the substrate 392 and the lid 393, to insert the end-treated HF 361 and SMF 371 from the guild holes 396 into this gap for end-to-end joining thereof in the V-groove, followed by removal of the wedges to hold, fix and connect the HF 361 and SMF 371 by means of the substrate 392 and the lid 393.
This allows the optical fiber connection portion 370 explained in FIG. 18 to be housed in the chassis 397 of the mechanical splice 391, thereby joining end-to-end the HF 361 and SMF 371.
In this manner, also in the case of use of the mechanical splice 391, since the cladding diameter of the HF 361 is equal to the cladding diameter of the SMF 371, the HF 361 and SMF 371 can be connected totally in the same way as the case of connecting typical SMFs to each other.
In the conventional optical fiber connection portion 370, however, the end-to-end joining of the HF 361 to the SMF 371 via the refractive index matching agent r7 causes a capillary phenomenon whereby the refractive index matching agent r7 penetrates into each air hole 364 of the HF 361 up to the depth of a few hundreds μm from the end face 361a. The refractive index of the cladding 363 is 1.458, and the refractive index of each air hole 364 is 1, but the refractive index matching agent r7 whose refractive index is 1.463 at room temperature penetrating into each air hole 364 would form 6 quasi-cores around the original central core 362.
For this reason, the effective core diameter of the HF 361 after connection, in other words, the diameter for allowing light propagation (the mode field diameter: MFD) becomes virtually larger than 9 μm before connection. Consequently, there is problem that the MFD difference between the HF 361 and SMF 371 makes their connection loss large.
For instance, in the case of use of the mechanical splice 391 explained in FIG. 20, joining the HF 361 and the SMF 371 would result in a large connection loss of approximately 0.85 dB at around room temperature and a wavelength of 1.55 μm. For comparison, the loss in connecting typical SMFs with the same core diameter to each other is around 0.1 dB.
Here, shown in FIG. 21 is a temperature characteristic of connection loss in a temperature range of −30° C. to +70° C. in the mechanical splice 391 after connection. In FIG. 21, the connection loss at room temperature after connection exceeds 0.8 dB, but is recovered to around 0.1 dB with increasing temperature.
The reason why the connection loss is recovered in the high-temperature range is as follows: As indicated by the temperature characteristic line 381 of FIG. 19, the refractive index of the refractive index matching agent r7 drops with increasing temperature and becomes equal to the refractive index of the cladding 363 at around 60° C. at which point the light confinement effect vanishes, and the connection loss equal to that of typical SMFs is thereby exhibited.
In the low-temperature range, on the other hand, as indicated by the temperature characteristic line 381 of FIG. 19, conversely, the refractive index of the refractive index matching agent r7 becomes large and therefore the refractive index difference between it and the cladding 363 increases, which increases the light confinement effect, which therefore also increases the light confinement effect of a virtual core formed by the original core 362 and the 6 air holes 364 with the refractive index matching agent r7 penetrated thereinto. This makes the MFD still larger than at room temperature, which thereby increases the MFD difference between the HF 361 and SMF 371 facing each other. The connection loss at −30° C. to 10° C. is as very high as around 1 dB.
There is therefore the problem that the conventional mechanical splice 391 causes large temperature characteristic variations of the connection loss, and particularly increases the connection loss in the low-temperature range.
Accordingly, it is a fourth object of the present invention to provide an optical fiber connection portion and an optical fiber splicer, which have a small connection loss and a small temperature characteristic variation of connection loss.
In the conventional optical fiber connection portion 370 and the mechanical splice 391, on the other hand, the amount reflected at the respective end faces 361a and 371a of the HF 361 and SMF 371 is required to be small.
Accordingly, it is a fifth object of the present invention to provide an optical fiber connection portion and an optical fiber splicer, which have a small connection loss and reflection amount, and small temperature characteristic variations of connection loss and reflection amount.